For conventional liquid crystal televisions, amorphous silicon TFTs (thin film transistors) are used in many cases and recognized as structures that can be relatively easily manufactured in terms of manufacturing cost.
However, under current moving image circumstances (e.g., 3D movies or 3D sports broadcast), it is difficult to clearly express moving images with liquid crystal televisions including amorphous silicon TFTs; accordingly, TFTs having high mobility that can respond at high speed are under development. For this reason, a microcrystalline silicon film has been developed.
It is known that the field-effect mobility (μFE_sat.) of a thin film transistor in which only a microcrystalline silicon film is employed for a channel formation region is higher than that of an amorphous silicon TFT; however, the crystallization may proceed excessively and the off-leakage current (Ioff) may be increased in such a transistor. Therefore, optimization of crystallinity is an issue.
The crystallinity of a microcrystalline silicon film can be increased, for example, under conditions in which monosilane (SiH4) and hydrogen (H2) are introduced into a parallel-plate plasma CVD (chemical vapor deposition) apparatus at an oscillation frequency of 13.56 MHz, the hydrogen flow rate is high, and the RF power is low.
Note that it is preferable to set the substrate temperature at film formation to approximately 200° C. to 300° C. for suppressing shrinkage (expression and contraction) of a glass substrate and increasing the gas decomposition efficiency at the time of plasma discharge.
In the case where a SiNx film is used as a gate insulating film of a thin film transistor and a microcrystalline silicon film serving as an active layer is formed over the SiNx film, the adhesion force between the microcrystalline silicon film and the SiNx film is low; as a result, a film bulge or film peeling tends to occur.
In the case of the above-described conventional conditions, the crystallinity tends to be low at an early growth stage of microcrystalline silicon and be improved as the deposition proceeds.
In order to solve these problems, in some cases, for example, N2O plasma treatment is performed on a surface of the SiNx film before forming the microcrystalline silicon film over the SiNx film for the purpose of securing reliability and adhesion at an interface between the active layer and the gate insulating film.
With such N2O plasma treatment, the adhesion between the microcrystalline silicon film and the SiNx film is somewhat improved but is still insufficient.
[Reference]
[Patent Document]
    [Patent Document 1] Japanese Published Patent Application No. 2006-279019    [Patent Document 2] Japanese Published Patent Application No. H06-132531